Question

When green light is incident on the surface of metal, it emits photoelectrons but there is no such emission with yellow colour light. Which one of the colours can produce emission of photo - electrons?
(A). Red
(B). Indigo
(C). Orange
(D). None of these

Answer 1

Hint: Study the concept and working of photoelectric effect. Try to understand the relation of photoelectron emission with wavelength or frequency of incident light on the metal. Consider the wavelength of different coloured light given in the question. We will get our answer.

Complete step by step answer:
Photoelectric emission can be defined as a phenomenon where when a light of suitable frequency is incident on a metal surface, electrons from the metal surface are ejected. These ejected electrons are called photoelectrons.
For photoelectric emission we need a minimum frequency of light, below which photoelectric emission doesn’t happen. This frequency is called the threshold frequency and the wavelength is called threshold wavelength.
Now, in the question we are given that the photoelectrons are emitted when green light is incident on the surface of the metal. But when yellow coloured light is incident on the metal surface photoelectric emission doesn’t happen. So, we can say that the frequency of green coloured light is the threshold frequency or near the threshold frequency, because photoelectric emission doesn’t happen with the yellow coloured light where the frequency of yellow light is less than the green light.
Now we know that, for photoelectric emission to happen the frequency of the incident light must be equal or above to the threshold frequency.
We know that the frequency of red and orange light is below the frequency of green light. so, photoelectric emission can not happen for red and orange lights.
But the frequency of indigo light is greater than the frequency of green light i.e. greater than the threshold frequency. So, we will get photoelectric emission for indigo light.
The correct option is (B).

Note: We can remember the increase or decrease in frequency of the light in the visible spectrum easily. In white light we have seven colours. It is in the order VIBGYOR. So, if we go from left towards right, the frequency of the light will decrease or the wavelength of the light increases.